Support a syntax for embedding QBS variables within command-line arguments.

QBS should support a syntax for passing the values of its own variables on the command line by expanding identifiers (i.e. alphanumeric + underscore, cannot start with digit) prefixed with '@' into the value of qbs.whatever

For example, @targetOS would be expanded to the value of qbs.targetOS.

As an example of a use case, perhaps a user wants their install directory to be named according to the platform and architecture. They should be able to write:

qbs install --install-root builds/@targetOS-@architecture

and, assuming they are running on a 64-bit Linux system, expect their build to be installed to builds/linux-x86_64.

Some things to think about:

• Do we support expansion of variables beyond just qbs.*?
• If we do allow this, are there potential problems with recursive definitions? i.e. can running a command with a @foo variable be dependent on the result of running such a command?
• How can we provide some sort of escaping syntax? Perhaps the user wants to literally embed @something in a command-line; they should have a way to do so - perhaps \@something or @@something
• If a variable is undefined should it be an error or return the string 'undefined'? I'd go for the former